In the field of industrial refrigeration systems, one of the major concerns is that of maintaining the optimum temperature in a cold storage room. Heat is removed from the storage compartment by an evaporator unit consisting of a fin coil heat exchanger and a fan motor for recirculating the air in the compartment. In most current cold storage systems, the evaporator fan motor operates continuously and the capacity controller on the system compressor is used to maintain the suction temperature, which in turn controls the evaporator temperature.
One of the major problems in virtually all refrigeration systems is the buildup of frost on the evaporator coils. Frost buildup is directly related to two factors: (1) moisture in the air which infiltrates into the system as a result of traffic in and out of the storage area; and (2) moisture deposits which are the result of evaporation from products stored in the system. As the frost builds up on the evaporator coil, it insulates the coil's surface thus creating a barrier to heat transfer and restricting air flow over the coils. The buildup of frost on the coils isolates the compressor capacity controller and thereby prevents it from sensing the actual room temperature.
Numerous methods have been used in the past to deal with this problem. One of the earliest methods for defrosting a refrigeration system was to mechanically remove the frost by hand. This method is impractical and uneconomic for large refrigeration systems, however, because it requires removal of the refrigerated items from the storage room.
Other methods for defrosting a refrigeration system include water defrosting, salt brine defrosting and glycol brine defrosting. Each of these methods employs a system for spraying a liquid solution over the heating coils to remove accumulated frost. Although these methods are relatively effective, they can cause corrosion of the evaporator coils and lead to maintenance problems. Furthermore, these methods are not economic for certain applications.
The most popular current methods for defrosting refrigeration systems operate by melting the frost on the surface of the evaporator, rather than washing or mechanically removing it therefrom. These methods typically use warm air, resistance heat or warm gas (usually waste heat from the system). The hot gas defrost system is particularly effective for large liquid expansion industrial refrigeration systems. However, the specific manner in which this defrosting method is typically used tends to result in inefficiency and unnecessary damage to the system.
In order to activate one of the above-mentioned defrost cycles, most current systems use a defrost timer clock sequencer designed only for very simple applications. This sequencer is a simple actuating system which initiates the defrost cycle at a predetermined time each day regardless of the temperature and humidity conditions present inside and outside of the refrigeration system. The use of such a simple defrost cycle, regardless of the system parameters, causes an increased load on the compressor and wastes energy. Furthermore, under certain conditions, such a timed defrost cycle will cause over-defrosting, while under other conditions it may provide inadequate defrosting. Either of these conditions causes undue wear on the system and leads to uneconomic operation.